Method and composition for stripping structural adhesives and the like

ABSTRACT

A MIXTURE IS COMPOSED OF CONCENTRATED NITRIC ACID AND A STRONG OXIDIZING AGENT, WITH AN OPTIMUM MIXTURE FORMULATION OF 86 PERCENT HNO3 (42* BE.) AND 7 PERCENT EACH OF H2O AND CRO3. THE MIXTURE CAN BE USED AS A BATH FOR ITEMS CONTAIMINATED WITH CURED STRUCTURAL ADHESIVES, PRIMERS AND PAINTS, AND THE LIKE, OR HAVE ADDED THERETO A FILLER TO PROVIDE A PAINT-LIKE MIXTURE SELECTIVELY APPLIED TO THE ITEMS WITH A BRUSH. IN THE CASE OF THE OPTIMUM MIXTURE, THE CONTAMINANTS ARE STRIPPED FROM THE ITEMS IN FROM ABOUT ONE-QUATER TO ABOUT ONE-HALF HOUR, WITH NO ADVERSE EFFECT ON THE SURFACES OF THE ITEMS. THE ITEMS ARE THEN THROUGHLY RINSED WITH WATER OR OTHER SOLVENT.

United States Patent 3,679,591 METHOD AND COMPOSITION FOR STRIPPING STRUCTURAL ADHESIVES AND THE LIKE Gilberto Carrillo, Riverside, Calif., assiguor to Rohr Corporation, Chula Vista, Calif. Y No Drawing. Filed Dec. 1, 1969, Ser. No. 881,328 Int. Cl.C11d 7/08 9, Claims ABSTRACT OF THEDISCLOSURE A mixture is composed of concentrated nitric acid and a strong oxidizing agent, with an optimum mixture formulation of '86 percent HNO (42.B.) and 7 percent each of H 0. and CrO The mixture can be used as a bath for items contaminated 'with cured structural adhesives, primers and paints, and the like, or have added thereto a filler to provide a paint-like mixture selectively applied to the items with a brush. In the case of the optimum mixture, the contaminants are stripped from the items in from about one-quarter to about one-half hour, with no adverse eifect on the surfaces of the items. The items are then thoroughly rinsed with water or other solvent. i 1

BACKGROUND OF INVENTION The present invention relates to new compositions of matter and their preparation, and more particularly to such compositions which are especially useful in strip- 3,679,591 Patented July 25, 1972 ice:

. DESCRIPTION OF INVENTION As previously stated, various attempts had been made to provide a suitable stripping composition for structural adhesives. However, the attempted compositions were inefiective, apparently because of being too mild. 'Experiments were carried out by applicant with chromic acid flakes and nitric acid which initially were not completely successful but upon the addition of water and with other changes resulted in the stripping composition of the present invention, as illustrated by the following representative examples.

EXAMPLE I Chromic acid flakes were mixed with nitric acid. A metal plate with cured modified epoxy adhesive'thereon was immersed in the mixture for several hours with some visible effect on the adhesive. It was observed that most ping cured structural adhesives, such as cured epoxies,

phenolics, and the like, from metal and metal alloy faces without adverse elfect on the surfaces. In the aircraft and aerospace industry, wide use is made of adhesives, paints and primers, for fabricating structural components or items, such as honeycomb panels of the chromic acid flakes went to the bottom of the container and did not go into solution.

7 \EXAMPLE II A mixture was prepared of: I

CF03 2m H O gm 25 HN'O3 (42 B.) (P 75 An aluminum plate with cured modified epoxy adhesive thereon was immersed in the mixture for three hours. The adhesive softened but did not strip away.

EXAMPLE III EXAMPLE. Iv

, Except for the use of .KMnO in place of CrO this example was similar to Example III, with substantially from the item, which adds to the cost thereof. Heretofore, the contaminants have been removed by grinding. Such methodof removal has not proved entirely satisfactory, the surface often being abraded or scratchedvin the process and so marred as tobe unacceptable,

Although various attempts have been made to provide a suitable compositionof matter for stripping the undesired adhesive and the like, none are known which have proved to be effective.

SUMMARY OF INVENTION similar immersion and end results.

' EXAMPLE V I EXAMPLE VI This example was similar to Example V except that I potassium permanganate was employed instead of the tremely effective in stripping cured structural adhesives chromic acid flakes, with a reduction in the number of specimens tested and greater selectivity with respect thereto. The application procedures and results were substantially the same as in the previous example.

EXAMPLE VII Mixed with 60 percent by Weight of concentrated nitric acid were 20 percent by weight each of chromic acid flakes and water. Test specimens with three to eight mil cured adhesive coatings required immersion up to two hours before the coatings were stripped.

Additional tests were performed varying the concentrated nitric acid from about 40 to about percent by weight, the weight of water and chromic acid flakes each beingwaried from about The several mixtures were applied totestspecimens-and contaminated structural parts having cured coatings of primers, paints and structural adhesives, such as epoxies, modified epoxies, nitrile epoxies, phenolics, nitrile phenolics, epoxy phenolics, nitriles and polyimides, varying in thickness. In the case of the lower nitric acid percentages,v the stripping took longer than for theQhigheijaer; centages. The thicker coatings also took longer than the thinnerones v 1 Application of the mixtures was, mainly by immersion. For more selective application or spotting, fused silica in powder form was added as a filler until the' mixture had 5 paint-like consistency and'was applied? Any of the mixtures with asuitable fille packed for applicationby extrusion.

The effect of the mixtureso'n the various test specimens was observed and it was" "determined thatsta'inlessa nd percent to about Bib percent. I

heat resistant steels, aluminum andtitanium, and alloys of'each','are not adversely. affected," if not treated-for eit tensively long periods bythe-mixtures.'Mild steels were attacked. it i 1 In the ease of great thicknesses of adhesives, it is rec--- ommended that they be ground down to about three or four mils, which can be easily done without danger of marring the metallic surface. This will reduce the amount of application time required and avoid possible harm to the surface of the item being treated. In any event,. the

item should be thoroughly rinsed with water or other suitablesolvent after treatment.

It was concluded that, for best results, the w'eights of the oxidizing agent and water should be equal, and that the optimum mixture is as expressed in the following example:

EXAMPLE VIII v Percent by wt. CrO; 7 H2O 7 mm, 42 B) va6 mixture is applied to an item contaminated with cured structural adhesives, primers"or paints. Tliick cO a'tihgS of contaminants are-ground down prior to 'application of the stripping mixture. After stripping, the 'iteriris' thor' oughly rinsed with water or other solvent.

I There has thus been provided a novel strippingtcoi'nposition which is especially useful in stripping cured structural adhesives, primers, paints, and the from aircraft structural parts contaminated thereby, without adverse elfect on the surfaces of the parts.

to fabOut -ZO percent by weighfof the mixture." I; r

Although the present invention has herein been de- What is claimed as new and useful and is desired tobe :35

secured by U.S.' Letters Patent is:

1. A method for stripping cured structural adhesives, primers,'paints,' and the-like, from metal items'contaminated thereby, comprising the steps of (a) Applying a mixture of concentrated nitric acid and a strong oxidizing agentto a contaminated metal item, said .applied mixture consisting essentially of from about percent to about percent by weight of nitric acid; (42?. Be) and from about lg 'percent to about 20 percent byweightof an,.oxidizing agent wselected from the-group consistingsof, chromic acid, and potassium permanganate, and the balance water;

-(b) keeping the applied mixture in contact withithe contaminated metal item until the contaminants have been stripped therefrom; and V N A (c) thereafter removing the applied mixture'from the 2. The method of claim 1 in which-the applied mixture isremovedby rinsing with water. V,

3; Themethod of claim 1 in whichthe oxidizing agent consistsessentially of equal weights oftchromicacid fiakes and-water.v v r ;4:,T-he.methodof claim 1 in which theoxidizing agent consists essentially of equal weights of potassium permanganate and water- 5.. A composition of matter consisting essentiallymf from about 40 percentto about 90 percent by weight of concentrated nitric .acid and from about 5 percent to about 35 percent each strong oxidizing agent selected from the group consisting of-'chromic"acid; and potassium permanganate, and the balance water. Y

6. The composition of 01a 5 in which the oxidizing agentincludes-a weight of water necessary'to put aselected group member in solution. r

::..7. The composition of claim 6 in which the weightof water equals the weight ofthe selected group member.

agent is chromic acid and consists essentially er equal weights ofCrO and'H O, together representingfrom about 10-percent to about 20 percent'by weight of-the mixture. t 1 t, i 1 9.-'The composition of claim 5 in'whic'h the oxidizing agent is made up of equal weights of"potassium'permanganate and water: and represents from abOut-IOLpercentQ I References Qited' UNITED "ST T S LPAT'ENTS,

2,762,694 9/1956 Newman 252'+'-142' X 2,762,728 a9/1956 Hahn 252' :101'X 2,883,311 4/1959 Halpert 134 -3 4/ 1968 Kendall 252-142 f OTHER REFERENCES- l I r 'Putilova, I. N. et al.: Metallic Corrosion Inhibitors",

Pergamon Press, New York, 1960, 15185 88.

D. ROSDOLPrimaryExaminer I W. E. scHULz, Assistant Examiner 1 

